With the improvement in the processing speed of computers or the like, in recent years, a clock frequency of CPU (central processing unit) and an operating clock frequency of CPU for reciprocating with external apparatus or devices have markedly increased.
The operating clock frequency increases as described above, whereby requirements for the performance of a printed wiring board for transmitting data signals between CPU and the external apparatus become severe.
More specifically, in a printed wiring board, it is required to match the characteristic impedance of a printed wiring circuit formed by a signal conductor to the characteristic impedance of another printed wiring circuit electrically connected to this printed wiring circuit or to match the characteristic impedance of a printed wiring circuit to the impedance of a circuit load electrically connected to this printed wiring circuit.
When mismatching is present between the characteristic impedances of printed wiring circuits electrically connected to each other or between the characteristic impedance of a printed wiring circuit and the impedance of a circuit load electrically connected to this printed wiring circuit, a part of data signals are reflected into a signal-transmitting source, whereby a signal finally directing to a circuit load is weakened to cause a problem that there is a possibility that transmission of desired data signals may not become infeasible. Such a problem becomes more marked to an unnegligible level as the operating clock frequency becomes higher.
It is thus indispensable to measure the characteristic impedance of a printed wiring circuit in a printed wiring board for the purpose of retaining the quality of the printed wiring board, and quality inspection of the printed wiring board is conducted on the basis of the measured result thereof.
A TDR (time domain refrectrometry) method has heretofore been used for measuring the impedance of a printed wiring circuit in a printed wiring board.
This method is a method that a pulse signal or step signal is transmitted to a transmission circuit composed of a signal circuit (circuit to be measured), which is an object of impedance measurement, and a reference ground circuit to detect a reflected signal within the transmission circuit, and a reflection coefficient found from the reflected signal is used to find an impedance value (characteristic impedance) of the transmission circuit (circuit to be measured).
In such a TDR method, a probe is used as an intervenor for electrically connecting a cable drawn out of a signal-transmitting source to the transmission circuit upon transmission of a signal to the transmission circuit.
Such impedance-measuring probes are roughly divided into those of 2 structures of a macrostrip structure formed by separately equipped with a contact pin for circuit to be measured for being brought into contact with a circuit to be measured and a contact pin for ground circuit for being brought into contact with a ground circuit and holding a plate-like dielectric layer between the contact pin for circuit to be measured and the contact pin for ground circuit, and a coaxial line structure formed by arranging an inner conductor and an outer conductor in a coaxial line form, drawing a contact pin for circuit to be measured out of the inner conductor and drawing a contact pin for ground circuit out of the outer conductor.
In such impedance-measuring probes, in any structure, the impedance measurement is conducted by bringing a leading end of the contact pin for circuit to be measured and a leading end of the contact pin for ground circuit into simultaneous contact with a signal circuit, which is a circuit to be measured, and a ground circuit, respectively.
In the conventional impedance-measuring probes, however, a conduction state is created by pressing the pointed contact pins against the signal circuit and ground circuit of a printed wiring board, so that the printed wiring board may be damaged upon measurement of its impedance.
In addition, since the contact pins made of a metal are brought into contact with the signal circuit and ground circuit of the printed wiring board, there is a problem that reliability on measurement is low in that a contact state between the impedance-measuring probe and the printed wiring board is unstable. It has thus been difficult to exactly measure impedance by the conventional impedance-measuring probe.
Operating clock frequencies of instruments for being connected to a computer are expected to become higher from now on, and electronic parts are considered to be fabricated more finely and at a higher density. Attending on this fact, it is considered that importance of exactly measuring characteristic impedance for surely retaining the quality of printed wiring boards increases more. However, there is a possibility that the conventional impedance-measuring probes may not sufficiently cope with such a demand.
On the other hand, as disclosed in, for example, Japanese Patent Application Laid-Open No. 183974/1991, it has heretofore been conducted to use an anisotropically conductive sheet as a member for achieving electrical connection in electrical inspection of printed wiring boards because contact stability is achieved, and moreover the printed wiring board can be prevented from being damaged upon contact, and to arrange this anisotropically conductive sheet between the printed wiring board and inspection electrodes to achieve a contact and conduction state.
However, the conventionally known anisotropically conductive sheets have involved such problems that a transmission loss becomes great when they are used in a high-frequency region, and so they have been unable to bring about sufficient properties in the impedance measurement in the high-frequency region, and have undergone difficulty in use in practice.